Various methods exist for concentrating particles in a fluid or separating particles from a fluid. In filtration, particles that are greater in size than the filter pore size are excluded or filtered out. Depth filters which do not have a specific pore size may trap particles in a filler matrix. Filtration is not easily implemented when the goal is to recover particles. In addition, filters may become clogged or caked which limits the usefulness of the filtration technique. Centrifugation is another method that may require the particles to have a different density or specific gravity than the fluid in which the particles are suspended. Adapting the centrifugation process for continuous processes may be complex and costly. Methods such as centrifugation, electrophoresis, and sedimentation may rely on unidirectional forces created by centripetal acceleration, electrostatic or electromagnetic fields, or gravity, respectively. These unidirectional forces may only allow particle migration in one direction. Chromatography is another method that relies on the selective retardation of some particles relative to the suspending flow. Some forms of industrial “scrubbers” separate particles by using elements that the particles stick or adhere to in order to trap the particles. For separating particles that have a higher density than the suspending fluid, inertial effects are often utilized, but such methods may require the suspending fluid to undergo an acceleration or change in flow direction. The particles, with their higher inertia do not always follow the flow and thus may be separated from the fluid. Some separation approaches used in micro fluidic systems configure flow paths or channels to influence the particle paths and exclude or direct particles away from some regions.
Obstacle based methods are other approaches that can be used in separation processes to circumvent the limitations of many of the other approaches for concentrating particles in a fluid or for separating particles from a fluid. For example, a field of obstacles may include surface coatings that bind to certain cell types. As a solution passes through the field in a micro-fluidic device, the specific cells bind to the obstacles and are immobilized, thus allowing the use of a fluoroscopic method to detect the cells of interest. Existing obstacle based separation techniques may use obstacles that shift obstacles in only one direction. Thus all particle-obstacle interactions result in a unidirectional shift relative to the fluid. Depth filters may utilize obstacles placed in a flow field to separate particles from a solution. When a solution initially enters a depth filter, the particles can pass though the filter, however, as the solution flows, the particles collect or deposit between the obstacles (e.g. fibers) either by adhesion or by jamming between the filter elements. Fibrous filters are another type of filter in which fibers are used for separation, which is achieved by the deposition of particles on collecting bodies.
It may be advantageous to separate particles in a process that is simple, efficient, and inexpensive and can be used in a continuous process that is less subject to clogging. It may be advantageous to collect particles suspended in a fluid instead of trapping or excluding them in a process that may be easily implemented in a broad variety of applications across a large range of processing scales.